RTNs家族研究进展

编码内质网蛋白家族的基因(RTNs)是一类广泛存在于真核生物的基因家族,它有着特殊的拓扑结构。RTNs蛋白的羧基端存在一个大约200个氨基酸的保守区(RHD),此保守区包含2个疏水区段。RTNs蛋白定位内质网膜上,高等脊椎动物中枢神经受损时,RTN4-A／Nogo-A起抑制神经生长的作用,RTNs蛋白家族其余成员的功能还不是很清楚,可能与胞内运输,细胞分裂和细胞凋亡等有关。现就RTNs成员的基本结构、表达分布、亚细胞定位、拓扑结构和RTNs蛋白的功能等进行综述。     

抑癌蛋白PTEN及PDZ蛋白对其的调节

pten基因是迄今为止发现的第1个具有双特异性磷酸酶活性的抑癌基因,该基因的编码产物PTEN蛋白,是具有蛋白与脂质磷酸酯酶活性的双特异性磷酸酯酶,作为1种重要的信号分子参与细胞增殖、分化、黏附、迁移、凋亡以及基因转录的调控. 最近,关于PTEN在信号转导中的作用以及细胞内PTEN的调节机制研究较多,尤其是PDZ蛋白对PTEN的调节作用. PTEN蛋白包括1个氨基端（N端）磷酸酯酶区域,1个与脂质结合的C2区域和1个含有PDZ结合序列的羧基端（C端）区域. PDZ结构域通过识别目标蛋白羧基端PDZ结合序列与目标蛋白相互作用,调控多种重要的细胞生理过程和信号传导途径.本文就抑癌基因pten编码产物PTEN蛋白的结构、PTEN的生物学功能和PDZ蛋白对PTEN调节的研究进展进行综述.     

RTN3与细胞凋亡

RTN3是RTN家族的成员之一,因其主要定位于内质网,所以用reticulon来命名.RTN3与RTN4B是RTN家族中目前已知的、唯一一对具有凋亡诱发功能的基因.过表达的RTN3介导了真核细胞的三大凋亡信号转导通路:死亡受体途径、线粒体途径、内质网途径,并使之交联形成凋亡调控网络;RTN3可与RTN4B相互作用,形成同源或异源二聚来调控细胞凋亡.另外,过表达RTN3还参与了细菌的类凋亡作用.RTN3广泛表达于多种组织,其过表达诱发凋亡的机制的总结将让人们更好的了解RTN3及其家族,完善细胞凋亡的信号转导研究.     

多功能蛋白质COMMD1

COMMD蛋白家族是近年来新发现的一组新颖的因子,它们结构独特,在生物进化过程中高度保守且有着某些共同的功能性质.它们广泛存在于多细胞生物中,最典型的特征是它们的羧基端存在一个高度保守而独特的结构-COMMD结构域,为蛋白质间的相互作用提供了关键界面.目前研究已证实,COMMD蛋白家族有10个成员,即COMMD1~10.COMMD1是COMMD蛋白家族中最先被证实且研究最为深入的蛋白质,在不同物种中广泛表达,且在人类的不同组织表达存在差异.COMMD1是1个多效性因子,参与许多生理活动,包括对铜代谢、转录因子NF-κB及低氧诱导因子1（HIF-1）的调节等.COMMD1参与铜的内稳态调节,是转录调节因子NF-κB及HIF-1的抑制物,通过对它们的抑制而介导多种基因的表达.本文就COMMD1的结构、表达、调节及生物效应等做一综述.     

间隙连接蛋白43羧基端在恶性肿瘤中的作用及机制

间隙连接蛋白43(connexin 43,CX43)是间隙连接蛋白家族的重要成员之一,参与体内众多生理和病理过程的调控。结构上,该蛋白由氨基端、跨膜结构及羧基端三部分组成,其羧基端上存在大量蛋白结合位点。通过这些位点,CX43能够与不同的蛋白发生相互作用:一方面,影响CX43自身的磷酸化状态,从而调控其降解、亚细胞定位以及装配等过程;另一方面,CX43羧基端还能够通过某些特定的结合位点,调控其他蛋白分子的功能状态,从而影响信号转导,调节细胞的生物学功能。近年来研究发现,该蛋白的羧基端(carboxyl terminal)显著地影响肿瘤细胞/肿瘤干细胞的生物学特性。该文就CX43羧基端的结构特点、与蛋白质的相互作用位点、调控肿瘤细胞/肿瘤干细胞增殖、迁移、自我更新和成瘤能力的作用机制进行简要综述。     

葡萄糖调节蛋白78研究进展

葡萄糖调节蛋白78（glucose regulated protein 78kD,GRP78）又称免疫球蛋白重链结合蛋白（immunoglobulin heavy chain binding protein,Bip）,是位于内质网上重要的分子伴侣,属热休克蛋白70家族的一员,GRP78分子及其DNA分子序列结构在许多生物物种中高度保守。GRP78在内质网中参与阻止内质网新生肽聚集、调节内质网钙稳态、抗内质网相关性细胞凋亡,以及启动未折叠蛋白反应等细胞生命过程。GRP78基因启动子上存在内质网应激反应元件（ERSE）和cAMP反应元件（CRE）等特殊的顺式作用元件,特异性转录因子ATF6等与GRP78启魂子上顺式作用元件发生动态结合,从而调节GRP78基础性或诱导性转录表达。近年来发现,GRP78与脂肪肝、肿瘤和神经系统等疾病的发生发展密切相关,GRP78生物学功能的研究已经引起生物学家们的广泛重视。     

玉米中Class Ⅰ和Class ⅡNAS蛋白之间的BiFC互作研究

烟草胺合成酶（nicotianamine synthase,NAS）能够催化合成植物体内铁运输所需的螯合物烟草胺（nicotianamine,NA）,在植物维持铁稳态方面发挥重要的作用。玉米、小麦和大麦等禾本科植物的NAS蛋白进化为ClassⅠ和ClassⅡ两个亚家族,可能分别参与调节铁的吸收和运输,其家族成员之间蛋白序列同源性较高,ClassⅡNAS具有特异的N端可变结构域。通过进化分析分析玉米NAS的两个亚家族,以及建模预测两类亚家族代表基因ZmNAS1(ClassⅠ)和ZmNAS3(ClassⅡ)的蛋白结构,结果表明ZmNAS1和ZmNAS3的三维结构高度相似,推测可能通过同源或异源二聚体化发挥功能;进一步通过双分子荧光互补（bimolecular fluorescence complementation,BiFC）分析ZmNAS1和ZmNAS3的相互作用,结果表明,ZmNAS1和ZmNAS3蛋白可以互作,删除N端可变结构域的ZmNAS3?N只能与ZmNAS3蛋白互作而不与ZmNAS1互作,推测ZmNAS可以形成同源二聚体,而形成异源二聚体需要ClassⅡ家族蛋白的N端可变结构域...     

大肠杆菌YggG与结合蛋白Era的相互作用研究

Era是细菌生长必须的一高度保守的GTPase。yggG是从大肠杆菌全基因组文库中钓取并克隆的Era结合蛋白基因,进一步的研究表明该基因在大肠杆菌中的表达与环境应激相关,提示yggG基因产物参与细菌的应激调控。为了阐明YggG蛋白与Era蛋白间的相互关系,利用所构建的双启动子表达载体pDH2-YggG-Ptac-Era在同一细胞中同时表达YggG与Era蛋白,并通过免疫共沉淀实验检测细菌裂解产物YggG与Era蛋白间的相互作用;在此基础上,构建并表达纯化了GST融合的Era蛋白氨基端截短肽和Era羧基端截短肽,通过GST Pull-down检测了Era不同功能区域与YggG蛋白间的相互作用。结果显示, Era/YggG 复合物仅存在于同时过表达Era和YggG蛋白的细菌细胞内,不诱导Era或者不诱导YggG蛋白过表达,均检测不到Era/YggG 复合物存在;纯化的GST不能Pull-down YggG蛋白,而纯化的GST融合的Era蛋白、Era氨基端截短肽及Era羧基端截短肽均可以Pull-down YggG蛋白;GST融合Era氨基端截短肽和GST融合的Era蛋白对YggG蛋白结合作用明显高于GST融合的Era蛋白羧基端截短肽。上述结果说明,YggG是一大肠杆菌Era结合蛋白,YggG与Era的氨基端和羧基端的结合活性存在差异。     

葡萄糖调节蛋白78研究进展

葡萄糖调节蛋白78(glucose regulated protein 78kD, GRP78)又称免疫球蛋白重链结合蛋白(immunoglobulin heavy chain binding protein, Bip),是位于内质网上重要的分子伴侣,属热休克蛋白70家族的一员,GRP78分子及其DNA分子序列结构在许多生物物种中高度保守.GRP78在内质网中参与阻止内质网新生肽聚集、调节内质网钙稳态、抗内质网相关性细胞凋亡,以及启动未折叠蛋白反应等细胞生命过程.GRP78基因启动子上存在内质网应激反应元件(ERSE)和cAMP反应元件(CRE)等特殊的顺式作用元件,特异性转录因子ATF6等与GRP78启动子上顺式作用元件发生动态结合,从而调节GRP78基础性或诱导性转录表达.近年来发现,GRP78与脂肪肝、肿瘤和神经系统等疾病的发生发展密切相关,GRP78生物学功能的研究已经引起生物学家们的广泛重视.     

细胞间隙连接的分子生物学研究进展

所有组成间隙连接的连接蛋白（Ｃｘ）结构，由４个跨膜片段、３个环及氨基端和羧基端组成。在不同Ｃｘ中各区域氨基酸序列同源性存在差异，并与其功能相关连。Ｃｘ基因由两个外显子、其间的一个内含子及某些调节元件组成。     

RTN4B interacting protein FAM134C promotes ER membrane curvature and has a functional role in autophagy

The endoplasmic reticulum (ER) is composed of a controlled ratio of sheets and tubules, which are maintained by several proteins with multiple functions. Reticulons (RTNs), especially RTN4, and DP1/Yop1p family members are known to induce ER membrane curvature. RTN4B is the main RTN4 isoform expressed in nonneuronal cells. In this study, we identified FAM134C as a RTN4B interacting protein in mammalian, nonneuronal cells. FAM134C localized specifically to the ER tubules and sheet edges. Ultrastructural analysis revealed that overexpression of FAM134C induced the formation of unbranched, long tubules or dense globular structures composed of heavily branched narrow tubules. In both cases, tubules were nonmotile. ER tubulation was dependent on the reticulon homology domain (RHD) close to the N-terminus. FAM134C plays a role in the autophagy pathway as its level elevated significantly upon amino acid starvation but not during ER stress. Moreover, FAM134C depletion reduced the number and size of autophagic structures and the amount of ER as a cargo within autophagic structures under starvation conditions. Dominant-negative expression of FAM134C forms with mutated RHD or LC3 interacting region also led to a reduced number of autophagic structures. Our results suggest that FAM134C provides a link between regulation of ER architecture and ER turnover by promoting ER tubulation required for subsequent ER fragmentation and engulfment into autophagosomes.     

Nogo/RTN4 isoforms and RTN3 expression protect SH-SY5Y cells against multiple death insults

Among the members of the reticulon (RTN) family, Nogo-A/RTN4A, a prominent myelin-associated neurite growth inhibitory protein, and RTN3 are highly expressed in neurons. However, neuronal cell-autonomous functions of Nogo-A, as well as other members of the RTN family, are unclear. We show here that SH-SY5Y neuroblastoma cells stably over-expressing either two of the three major isoforms of Nogo/RTN4 (Nogo-A and Nogo-B) or a major isoform of RTN3 were protected against cell death induced by a battery of apoptosis-inducing agents (including serum deprivation, staurosporine, etoposide, and H₂O₂) compared to vector-transfected control cells. Nogo-A, -B, and RTN3 are particularly effective in terms of protection against H₂O₂-induced increase in intracellular reactive oxygen species levels and ensuing apoptotic and autophagic cell death. Expression of these RTNs upregulated basal levels of Bax, activated Bax, and activated caspase 3, but did not exhibit an enhanced ER stress response. The protective effect of RTNs is also not dependent on classical survival-promoting signaling pathways such as Akt and Erk kinase pathways. Neuron-enriched Nogo-A/Rtn4A and RTN3 may, therefore, exert a protective effect on neuronal cells against death stimuli, and elevation of their levels during injury may have a cell-autonomous survival-promoting function.     

Reticulon 3 is involved in membrane trafficking between the endoplasmic reticulum and Golgi

Reticulons (RTNs) constitute a family of endoplasmic reticulum (ER)-associated proteins with a reticular distribution. Despite the implication of their neuronal isoforms in axonal regeneration, the function of their widely expressed isoforms is largely unknown. In this study, we examined the role of the ubiquitously expressed RTN3 in membrane trafficking. Ectopically expressed RTN3 exhibited heterogeneous patterns; filamentous, reticular, and granular distributions. The ER morphology changed accordingly. In cells where RTN3 displayed a filamentous/reticular distribution, protein transport between the ER and Golgi was blocked, and Golgi proteins were dispersed. In contrast, ERGIC-53, a marker for the ER-Golgi intermediate compartment, accumulated at the perinuclear region, and remained there even after cells were treated with agents that induce redistribution of Golgi proteins to the ER, indicating an inhibition of Golgi-to-ER transport of ERGIC-53. These results suggest that RTN3 plays a role in membrane trafficking in the early secretory pathway.     

Five Arabidopsis Reticulon Isoforms Share Endoplasmic Reticulum Location,Topology, and Membrane-Shaping Properties

The cortical endoplasmic reticulum (ER) in tobacco (Nicotiana tabacum) epidermal cells is a network of tubules and cisternae undergoing dramatic rearrangements. Reticulons are integral membrane proteins involved in shaping ER tubules. Here, we characterized the localization, topology, effect, and interactions of five Arabidopsis thaliana reticulons (RTNs), isoforms 1-4 and 13, in the cortical ER. Our results indicate that RTNLB13 and RTNLB1-4 colocate to and constrict the tubular ER membrane. All five RTNs preferentially accumulate on ER tubules and are excluded from ER cisternae. All isoforms share the same transmembrane topology, with N and C termini facing the cytosol and four transmembrane domains. We show by Förster resonance energy transfer and fluorescence lifetime imaging microscopy that several RTNs have the capacity to interact with themselves and each other, and we suggest that oligomerization is responsible for their residence in the ER membrane. We also show that a complete reticulon homology domain is required for both RTN residence in high-curvature ER membranes and ER tubule constriction, yet it is not necessary for homotypic interactions.     

Nogo and its paRTNers

Reticulons (RTNs) are a relatively new eukaryotic gene family with unknown functions but broad expression and peculiar topological features. RTNs are widely distributed in plants, yeast and animals and are characterized by a approximately 200-amino-acid C-terminal domain, including two long hydrophobic sequences. Nogo/RTN4 can inhibit neurite growth from the cell surface via specific receptors, whereas more general, 'ancestral', RTN functions might relate to those of the endoplasmic reticulum - for example, intracellular trafficking, cell division and apoptosis. Here, we review the taxonomic distribution and tissue expression of RTNs, summarize recent discoveries about RTN localization and membrane topology, and discuss the possible functions of RTNs.     

Two hydrophobic segments of the RTN1 family determine the ER localization and retention

Reticulon (RTN) proteins are localized to the endoplasmic reticulum (ER), and are related to intracellular membrane trafficking, apoptosis, inhibiting axonal regeneration, and Alzheimer's disease. The RTN proteins are produced without an N-terminal signal peptide. Their C-terminal domain contains two long hydrophobic segments. We analyzed the ER localization signal of human RTN1-A. Mutant proteins lacking the first (39 residues) or second (36 residues) hydrophobic segment showed ER localization. On the other hand, the mutant lacking both hydrophobic segments was cytosolic. Enhanced green fluorescent protein (EGFP) tagged with the first or second hydrophobic segment of RTN1-A was localized to the ER. These results suggest that each hydrophobic segment determines the ER localization. In addition, EGFP tagged with the truncated form of the first hydrophobic segment exhibited the localization to the Golgi rather than the ER. This suggests that the length of the hydrophobic segment contributes to the ER retention of RTN1-A.     

Caenorhabditis elegans reticulon interacts with RME-1 during embryogenesis

Reticulon (RTN) family proteins are localized in the endoplasmic reticulum (ER). At least four different RTN genes have been identified in mammals, but in most cases, the functions of the encoded proteins except mammalian RTN4-A and RTN4-B are unknown. Each RTN gene produces 1-3 proteins by different promoters and alternative splicing. In Caenorhabditis elegans, there is a single gene (rtn gene) encoding three reticulon proteins, nRTN-A, B, and C. mRNA of nRTN-C is expressed in germ cells and embryos. However, nRTN-C protein is only expressed during embryogenesis and rapidly disappears after hatch. By yeast two-hybrid screening, two clones encoding the same C-terminal region of RME-1, a protein functioning in the endocytic recycling, were isolated. These findings suggest that nRTN-C functions in the endocytic pathway during embryogenesis.     

Mapping of interaction domains mediating binding between BACE1 and RTN/Nogo proteins

BACE1 is a membrane-bound aspartyl protease that specifically cleaves amyloid precursor protein (APP) at the beta-secretase site. Membrane bound reticulon (RTN) family proteins interact with BACE1 and negatively modulate BACE1 activity through preventing access of BACE1 to its cellular APP substrate. Here, we focused our study on RTN3 and further show that a C-terminal QID triplet conserved among mammalian RTN members is required for the binding of RTN to BACE1. Although RTN3 can form homo- or heterodimers in cells, BACE1 mainly binds to the RTN monomer and disruption of the QID triplet does not interfere with the dimerization. Correspondingly, the C-terminal region of BACE1 is required for the binding of BACE1 to RTNs. Furthermore, we show that the negative modulation of BACE1 by RTN3 relies on the binding of RTN3 to BACE1. The knowledge from this study may potentially guide discovery of small molecules that can mimic the effect of RTN3 on the inhibition of BACE1 activity.     

Analysis of the reticulon gene family demonstrates the absence of the neurite growth inhibitor Nogo-A in fish

Reticulons (RTNs) are a family of evolutionary conserved proteinswith four RTN paralogs (RTN1, RTN2, RTN3, and RTN4) presentin land vertebrates. While the exact functions of RTN1 to RTN3are unknown, mammalian RTN4-A/Nogo-A was shown to inhibit theregeneration of severed axons in the mammalian central nervoussystem (CNS). This inhibitory function is exerted via two distinctregions, one within the Nogo-A–specific N-terminus andthe other in the conserved reticulon homology domain (RHD).In contrast to mammals, fish are capable of CNS axon regeneration.We performed detailed analyses of the fish rtn gene family todetermine whether this regeneration ability correlates withthe absence of the neurite growth inhibitory protein Nogo-A.A total of 7 rtn genes were identified in zebrafish, 6 in pufferfish,and 30 in eight additional fish species. Phylogenetic and syntenicrelationships indicate that the identified fish rtn genes areorthologs of mammalian RTN1, RTN2, RTN3, and RTN4 and that severalparalogous fish genes (e.g., rtn4 and rtn6) resulted from genomeduplication events early in actinopterygian evolution. Accordingly,sequences homologous to the conserved RTN4/Nogo RHD are presentin two fish genes, rtn4 and rtn6. However, sequences comparableto the first 1,000 amino acids of mammalian Nogo-A includinga major neurite growth inhibitory region are absent in zebrafish.This result is in accordance with functional data showing thataxon growth inhibitory molecules are less prominent in fisholigodendrocytes and CNS myelin compared to mammals.